Mythical creatures still found on island (5)
I believe the answer is:
yetis
'mythical creatures' is the definition.
(yeti is a kind of mythical creature)
'still found on island' is the wordplay.
'still' becomes 'yet' ('yet' can be a synonym of 'still').
'found on' means one lot of letters go next to another.
'island' becomes 'is'.
'yet'+'is'='YETIS'
(Other definitions for yetis that I've seen before include "Abominable snowmen" , "Mythical creatures" , "Rarely seen group of creatures" , "Beasts that aren't real" , "More than one mythic creature" .)